This invention relates to a magnetic variation sensor which can be used as a rotational speed sensor such as a vehicle wheel speed sensor or an engine revolution sensor, and a method of manufacturing such a sensor.
A vehicle anti-lock brake system (ABS) needs wheel speed sensors to detect wheel speeds. Typical such wheel speed sensors are magnetic variation sensors. FIG. 7 shows a magnetic variation sensor comprising a sensor unit a for picking up variations in the magnetic flux produced by a ring member B mounted on a portion of a vehicle that rotates together with a vehicle wheel and converting such variations in the magnetic flux into electric signals, and a mounting portion b at which the sensor P is mounted to a stationary part of the vehicle near the ring member B. The sensor unit a may be a Hall IC 1 comprising a Hall element (magneto-electricity conversion element) and a signal processing circuit, or may comprise a coil wound around a bobbin, a pole piece inserted in the bobbin, and a magnet provided at the rear end of the pole piece, as disclosed in (unexamined) JP patent publication 2001-174471.
If the wheel speed sensor P is of the type including the Hall IC 1, the Hall IC picks up variations in the bias magnetic field produced by the ring member B when the ring member B rotates together with the vehicle wheel and converts such variations in the magnetic field into electric signals. In order to produce such variations in the magnetic field corresponding to the rotational speed of the ring member B and thus the vehicle wheel, the sensor P has a magnet and the ring member B is formed of a non-magnetized ferromagnetic (such as iron) encoder. Alternatively, the ring member or encoder B is magnetized such that N and S poles circumferentially alternate with each other. In this case, the magnet of the sensor P is omitted.
As shown in FIG. 7, with a cable 4 connected to the sensor unit a, a resin shell 5 is formed by molding so as to enclose the sensor unit a and the portion of the cable 4 connected to the sensor unit a. When forming the shell 5 by molding a resin, a portion of the resin is formed into a mounting piece 6 that is integral with the shell 5. The sensor P is adapted to be mounted to a vehicle by fastening the mounting piece 6 to a stationary part D of the vehicle by means of e.g. a screw. If the screw is in direct engagement with the mounting piece 6, which is made of a resin, the screw cannot be tightened with sufficient force because the resin forming the mounting piece 6 is low in strength. Thus, (unexamined) JP patent publication 2000-171475 proposes to form a screw (or bolt) hole by embedding a metallic ring 16 in the resin when forming the mounting piece 6 by molding.
The mounting piece 6 is inclined at a predetermined angle with respect to the height direction (direction perpendicular to the plane containing the sheet of FIG. 7) of the sensor unit a according to the position of a stationary part of the vehicle to which the sensor P is to be mounted.
In this arrangement, the shell 5 and the mounting piece 6 cannot be formed simultaneously but have to be formed in separate stages as disclosed in JP patent publication 2000-171475. The manufacturing cost is thus high.
JP patent publication 2003-307523 (unexamined) proposes to form the shell 5 by molding a resin with a portion of a separate member as the mounting piece 6 embedded in the resin so as to form a predetermined angle with respect to the thickness direction of the sensor unit a.
The mounting piece 6 can thus be secured to the shell 5 simultaneously when the shell 5 is formed by molding. But in this arrangement, it is necessary to provide a metallic case in the mold assembly to protect the sensor unit a and also to hold the separate mounting piece 6 in a predetermined angular position when forming the shell by molding a resin. The mold assembly is thus expensive.
An object of the invention is to provide an improved magnetic variation sensor and a method of manufacturing a magnetic variation sensor which makes it possible to form a mounting piece having any desired angle with respect to the shell at a low cost.